1. Field of the Invention
The present invention relates to a production method of polyamides employing a rapid estimation of the mole balance of polyamides during a melt polymerization process, and more specifically, relates to a production method of polyamides by melt polymerization in a batch polymerization vessel employing a rapid estimation of the mole balance of polyamides under melt polymerization from a melt viscosity during melt polymerization process. The present invention further relates to a production method of polyamides by further solid phase-polymerizing a melt-polymerized polyamide, more specifically, relates to a production method of solid phase-polymerized polyamides by the solid phase polymerization of melt-polymerized polyamides in which properties of the starting melt-polymerized polyamides are rapidly and easily estimated to determine solid phase polymerization conditions (time, temperature, pressure) from the estimated results. The present invention still further relates to a method of easily estimating the melt viscosity, more specifically, relates to a method of calculating the melt viscosity of polyamides from a stirring torque of a melt polymerization system for producing polyamides in a batch polymerization vessel.
2. Description of the Prior Art
Generally, a melt-polymerized polyamide is produced by the dehydrating polycondensation of a diamine monomer and a dicarboxylic acid monomer, in which the monomers are melt-polymerized at reaction temperatures higher than the melting point of polyamide being produced. In the production of polyamide, it is important to maintain the preset mole balance and the preset polymerization conditions to ensure the production of polyamide with uniform and stable quality. Therefore, various methods have been employed to maintain the preset values of mole balance, polymerization time, polymerization temperature, polymerization pressure, etc.
The mole balance is a particularly important process factor to be precisely controlled, because it has a significant influence on the polymerization degree of melt-polymerized polyamide. In batch polymerization method, however, the escape of diamine out of the reaction system during polymerization is difficult to avoid and it is quite difficult to make the escaped amount always uniform from batch to batch, even if the preset mole balance at the initial charge of monomers is the same in every batch and the polymerization conditions are made uniform from batch to batch. Therefore, the mole balance of polyamide to be produced is out of control and actually depends how the polymerization proceeds.
If the mole balance is not uniform from batch to batch, the polymerization time should be adjusted depending on the batch-to-batch variation of mole balance to make the polymerization degree of resultant polyamide uniform from batch to batch. In other words, if the mole balance varies from batch to batch, the real time observation of polymerization degree, if possible, would enable the discharge of polyamide at a stage where the aimed polymerization degree is reached, thereby making the polymerization degree of the resultant polyamide stable from batch to batch. The batch-to-batch stability of polymerization degree of polyamide would result in the batch-to-batch stability of flow characteristics, such as melt viscosity, molecular weight and relative viscosity of molten polyamide, which affect the moldability and processability.
The mole balance is generally determined by titration analysis which takes several hours because a polyamide which is taken out of a melt polymerization vessel and then solidified is used as the analyzing sample. The sample is dissolved into a specific solvent and subjected to neutralization titration to determine the concentration of terminal carboxyl and the concentration of terminal amino. From the titration results, the mole balance is calculated. Therefore, it is difficult to feed back the measured results of the mole balance in a preceding batch to the next batch before determining the production conditions. If the mole balance can be quickly determined during melt polymerization, a measure quite effective for determining the subsequent melt polymerization conditions of the same batch or the next batch is obtained, thereby providing an industrially significant method.
Japanese Patent Publication No. 48-36957 proposes a real-time measurement of properties by a viscometer in the continuous production of polyamide. In the proposed method, the melt viscosity which is related to the polymerization degree can be quickly determined during melt polymerization without using chemical analysis. However, the document is quite silent as to the measurement of the mole balance. A capillary viscometer is taught to be generally used. In a batch polymerization, however, the use of such a viscometer is expected to cause severe difficulty which is not encountered by a continuous polymerization. The content of reaction liquid changes from the starting monomers to polyamide just before discharge, and its viscosity may change from 0.01 to about 500 Pa·s. If the solubility of the starting material to polyamide is not sufficient, the capillary is clogged to make a stable use of viscometer difficult. In addition, the viscometer is generally an expensive measuring device to increase equipment cost.
Recently, an on-line measurement of polymer properties using a near infrared spectrometer has been proposed. Near infrared ray is quite suitable for non-destructive analysis and real time analysis because of its transmitting properties superior to ultraviolet ray and infrared ray. Problems in stability of light source, spectrometry system, detector, and hard ware and soft ware for computing spectra has prevented it from being put into practical use. Recent development in the technique has solved many of the problems and the near infrared spectrometer has come into the market.
Production methods of polyester in which properties of polyester are measured using a near infrared spectrometer and the reaction conditions are controlled by the measured values are disclosed, for example, in Japanese Patent Application Laid-Open Nos. 2-306937, 10-182802, 11-60711 and 11-315137. Japanese Patent Application Laid-Open No. 6-322054 proposes a production method of phenol resin in which the chemical composition of the system is measured by a near infrared spectrometer and the reaction is continued by estimating the progress of reaction on the basis of measured values. WO 96/05498 discloses a method of controlling the amount of solvent in a reaction solution by measuring the concentration of a solution comprising an amide solvent and an aromatic polyamide using a near infrared spectrometer. WO 96/16107 discloses a production method of polyamide by a continuous melt polymerization employing a near infrared spectrometer. In the proposed method, the concentrations of carboxyl end group (terminal carboxyl) and amino end group (terminal amino) are measured. The feeding amount of diamine is controlled on the basis of the measured results to control the ratio between the concentrations of carboxyl end group and amino end group, thereby attaining the aimed mole balance and preventing the formation of solid matters in the reaction apparatus.
To obtain precise measured results by a near infrared spectrometer, the solution under measurement should be homogeneous, requiring the removal of bubbles in the solution. However, the removal of bubbles are accompanied by a considerable difficulty in the production of polyamide, because it involves the elimination of water. In addition, since the lower limit for the measurement by a near infrared spectrometer is close to the controlling range of mole balance to be required for polyamide, it is questionable whether the measured results are effective information. Further, a near infrared spectrometer is expensive and the installation thereof into a polymerization vessel requires a considerable cost in the conversion of polymerization vessel, etc.
Generally, as a molding polyamide, a polyamide with low viscosity is used because it is molded by injection molding and so required to be highly flowable in molten state. In the applications to bottle, sheet, film and fiber, polyamide is molded also by extrusion in addition to injection molding. Therefore, polyamide with moderate to high viscosity is used in the applications to bottle, sheet, film and fiber because a flowability in molten state lower than that of the molding polyamide is required.
As the low viscosity polyamide for use mainly as molding material, a melt-polymerized polyamide is used as it is or after drying. If the moderate to high viscosity polyamide for use mainly as bottle, sheet, film or fiber is intended to produce by melt polycondensation, a specific polymerization apparatus is required because a stirring device commonly used cannot generate a stirring force enough to maintain the uniform molten state in the polymerization vessel. If the polycondensation is continued from a low viscosity until a moderate to high viscosity is reached, the time for maintaining the molten state (reaction time) is prolonged to cause damage of polyamide molecule (degradation of polymer molecule due to radical generation) and abnormal reaction such as non-linear molecule propagation (formation of three-dimensional polymer), this in turn increasing the formation of gel and fish eye to invite practical disadvantage. Therefore, the moderate to high viscosity polyamide has been produced by a solid phase polymerization in which a low viscosity polyamide is first produced by a melt polycondensation and then heat-treated in solid phase.
The solid phase polymerization of polyamide is generally conducted by calculating the increase of polymerization degree during the solid phase polymerization from solid phase polymerization temperature, time and pressure using a rate equation of amidation reaction while taking the mole balance and properties relating to the polymerization degree such as molecular weight and relative viscosity of the starting polyamide into consideration, by determining the conditions of solid phase polymerization on the basis of the calculated results, and by terminating the solid phase polymerization when the aimed polymerization degree is reached. The mole balance of the starting polyamide is an important property to be surely taken into account because it significantly affects the increasing speed of the polymerization degree. To evaluate the increasing polymerization degree during the solid phase polymerization, the polymerization degree of the starting polyamide is necessary. Thus, the analysis of the properties relating to the polymerization degree such as molecular weight and relative viscosity is required before solid phase polymerization.
A solid phase polymerization without analyzing the starting polyamide is known, in which polyamide under solid phase polymerization is sampled and rapidly measured on its melt viscosity, etc., and the polymerization degree during the solid phase polymerization is estimated from the measured results thereby to determine the end point of the solid phase polymerization. However, since the solid phase polymerization of polyamide is of high reaction rate as compared with polyester, the solid phase polymerization of polyamide has limited time for determining its end point, this making the operation restless.
If the mole balance and polymerization degree of melt-polymerized polyamide to be used as the starting material of the solid phase polymerization are always constant, the analysis is not required for each time. As mentioned, however, the mole balance of melt-polymerized polyamide depends on the progress of the polymerization. The batch-to-batch variation of mole balance results in the batch-to-batch variation of polymerization degree (molecular weight, relative viscosity, etc.) which is significantly affected by the mole balance.
The mole balance, molecular weight and relative viscosity of melt-polymerized polyamide are generally chemically analyzed. The mole balance and number average molecular weight are determined, for example, from the calculation using the measured values which are obtained by the measurement of a carboxyl end concentration and an amino end concentration by neutralization titration of a solution of polyamide in a specific solvent. The relative viscosity is determined by dividing a dropping time (second) of a polyamide solution in concentrated sulfuric acid measured using a viscometer by a dropping time (second) of sulfuric acid itself. These chemical analyses usually require 2 to 4 h until the results are obtained. Therefore, the melt-polymerized polyamide should be stored in a silo, etc. before solid phase polymerization until the results of analyses are obtained, thereby preventing the efficient production.